It is now common practice to explore the oceans of the earth for deposits of oil, gas and other valuable minerals by seismic techniques in which an exploration vessel imparts an acoustic wave into the saltwater, typically by use of a compressed air "gun". The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels back upwardly where it is detected by microphone or "hydrophone" elements in a streamer or array towed by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed acoustic array comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof sheath and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the towed array is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. Conductors carry the signals from the hydrophone elements to the recording equipment (so-called "acoustic data").
In addition to acoustic data, it is also important to collect and transmit data concerning operational status of the array to the vessel (so-called "nonacoustic data"). Nonacoustic data comprises physical characteristics of interest.
For example, during operation, the towed array is surrounded by saltwater. A hydrophone element is a high impedance device, therefore any saltwater coming into contact with the element causes leakage paths for electrical current present in the leads thereto, either severely distorting the signal produced by the hydrophone element or shorting the hydrophone element entirely. Other circuitry within the array may malfunction due to contact with saltwater. Therefore, it is very important to keep the hydrophone element dry and to notify persons on the vessel should saltwater invade any of the modules in the array.
Further, it may be important to know that electric power delivered to components within the array is of the appropriate voltage or amperage to ensure proper operation of the components.
It may also be important to know the depth at which the array is operating and the temperature of the water surrounding the array to be able to judge the acoustic data properly.
From one job to another however, these physical characteristics may vary in importance. For instance, it may be critical in one application to know the temperature of the array and utterly inconsequential in another application. Thus, although a manufacturer may have a standard design for an array, the design may have to be modified for each individual array produced, depending upon what nonacoustic sensors are required within the array.
At one time, the analog signals output by the hydrophones were transmitted to the vessel where they were digitized and stored for later processing and analysis. In such systems, totally separate analog data channels were employed for acoustic and nonacoustic data. Since these analog channels were potentially very long (on the order of miles) and since interference from stray electromagnetic interference within the array is always present to a degree, the data transmission rates on these analog channels were relatively low.
Today, it is possible to construct towed arrays having digital data channels. With digital data transmission, data transmission rates are higher and, with proper attention to electromagnetic interference, data fidelity is maintained from the hydrophone to the recording equipment.
For instance, U.S. Pat. No. 3,996,553, that issued on Dec. 7, 1976 is directed to a plurality of data acquisition units connected to a central signal processor through a common telemeter link. The telemeter link includes a data channel, an interrogation channel and a control channel. The central signal processor sends an interrogation signal through the interrogation channel to the data acquisition units. As each data acquisition unit recognizes the interrogation signal, it transmits its acquired data back up to the central processor through the data channel. Any selected data acquisition unit, when it receives a control signal through the control channel at the same time that it receives an interrogation signal through the interrogation channel, can be caused to perform a function different from all other units. The signal propagation velocity through the control channel is different from the signal propagation velocity through the interrogation channel. One of the two signals may be transmitted through the faster channel at a selected time later than the other of the two signals is transmitted through the slower channel. The selected time difference between the transmission of the two signals is proportional to the ratio of signal propagation delay difference between channels. Accordingly, the signal propagating through the faster channel will overtake and intercept the signal propagating through the slower channel at the selected data acquisition unit. Each data acquisition unit may have one or more input channels. Each input channel is connected in turn to the data channel through a stepping switch or multiplexer. The interrogation signal may exist in one of two or more states. In the first state, the interrogation signal resets the multiplexer, in the second state, the interrogation signal advances the multiplexer to the next input channel in sequence.
Thus, this system allows for control signals to command changes of state within the individual multiplexers in the system.
In a similar vein, U.S. Pat. No. 4,092,629, that issued on May 30, 1978, is directed to a seismic sensor cable assembly having 50 cable sections (or modules) and much of the seismic data processing electronics decentralized into the cable structure itself. The cable assembly is coupled to a central station mounted in a recording vehicle. The central station includes recording circuitry and apparatus to receive, process and record digital data words from a data link in the cable assembly and circuity for transmitting control signals into an interrogation link in the cable assembly. The electrical output of each sensor unit constitutes a separate input channel. The cable sections are spaced apart and interconnected by small diameter, cylindrical connector modules (not to be confused with the modules containing the hydrophones) that contain a transceiver unit for processing the signals from ten sensor units in an associated cable section contained within each transceiver unit is a multiplexer having a plurality of filtered input channels coupled respectively to the elemental sensor units, and an output. In response to a first interrogation pulse transmitted through the interrogation link from the central station unit, the multiplexer advances to a selected input channel to acquire a first analog data sample. A second interrogation pulse sequences the respective multiplexers in all 50 modules to select a second channel for sampling and digitizing to provide digital data words for the respective second channels. The self-clocking phase-encoded data words transmitted from the respective transceiver associated with each cable section are ordered in accordance with the propagation delay time of the interrogation link between the central station and the respective transceiver units. Self-clocking data words from corresponding channels within the respective transceiver units are ordered in accordance with the channel-select sequence during a scan cycle. Although there is a provision for nonacoustic data to be sensed and placed along with acoustic data into the data channel, there is no circuitry for allowing preprogramming of the number, characteristics and order of nonacoustic sensors to be used in the array.
Unfortunately, most arrays constructed to this point still employed separate channels for transmission of acoustic and nonacoustic data. These arrays suffered the extra cost, weight and reliability penalties brought about by complicating the structure. Even arrays having unified acoustic and nonacoustic data channels, whether digital or analog, were nonprogrammable.
What is needed in the art is a towed array having a capability to more efficiently integrate acoustic and nonacoustic data into a single digital channel. Toward that end, what is needed is a circuit for collecting and preparing such nonacoustic data for transmission along with the acoustic data in a synchronized form. Finally, the prior art has failed to provide a flexible nonacoustic data collection circuit, one that is programmable to take into account different sensor types, numbers and sensing orders.